The present invention pertains generally to liquid crystal microdisplays systems and, more particularly, to achieving higher contrast in microdisplay systems having a twisted nematic cell design.
Microdisplays are the most recent addition to the family of flat-panel displays. While microdisplays are based on a number of different techniques to generate modular light, all are based on the use of microfabrication technologies to produce a rectangular array of pixels on a semiconductor back plane. Examples of microdisplays include liquid crystal displays, field emission displays, and digital micro-mirror displays.
At present, liquid crystal display (LCD) devices have found varieties of applications as a thin full color display. The very first types of LCDs used DSM (dynamic scattering mode), but twisted nematic (TN) mode has become the standard today. Almost all active matrix drive displays use TN type LCDs. A typical TN device comprises nematic liquid crystal sandwiched between two substrates at least one of which is transparent. Transmissive TN devices comprise two glass substrates. A special surface treatment is given to each glass substrate such that the molecules are parallel to each substrate yet the director at the top of the device is perpendicular to the director at the bottom. This configuration sets up a 90xc2x0 twist into the bulk of the liquid crystal, hence the name twisted nematic display. The amount of the twist can be varied by changing the treatment angle given to each substrate. Different twist angle values give significantly different optical properties. Typical twist angles range from 45xc2x0 to 270xc2x0 depending on application. In a typical reflective type TN display used for microdisplays, director at the bottom is rotated 45xc2x0 from the director at the top. The light will pass through the liquid crystal before and after being reflected off a pixel surface on bottom substrate of the device.
The underlying principle in a normally black reflective TN display is the manipulation of polarized light. If no voltage is applied, the liquid crystal (LC) molecules of the cell are aligned parallel to the alignment surfaces. Before entering the cell, light passes through a polarizer that is aligned with the LC molecules on the top surface. When polarized light enters the cell, its polarization changes so that immediately prior to being reflected, the light has nearly circular polarization. After being reflected, the light reverses its direction and emerges from the cell in nearly the same polarization state in which it entered the cell. In a normally black TN LCD, an analyzer, rotated by 90xc2x0 with respect to the polarizer, is placed in the output path of the light reflected from the LC cell. Because the analyzer is rotated 90xc2x0, light will not pass through the analyzer when the cell is in off state. In the bright state or the on state of the device, LC molecules tend to orient with the applied electric field. The light emerging from the cell is therefore rotated nearly 90xc2x0 from the polarized light entering the cell. Because the exiting light is rotated close to the analyzer direction, most of the light will pass through the analyzer only when the cell is on.
Reflective LCD microdisplays are used in a many projection and virtual view applications. These applications include: multimedia front projectors, rear-projection computer monitors, rear-projection televisions, and near-to-the-eye (NTE) displays. Light valves that are reflective provide important advantages in projection displays. Controlling circuitry placed below the mirror surface does not obstruct the clear aperture. More advanced IC technology is available for substrate materials that are opaque, and a more compact system may be achieved when the reflected output beam is folded back on the input. One particular type of reflective LC technology, the liquid-crystal-on-silicon (LCoS) microdisplay, is emerging as an attractive choice for such applications. The advantage of LCoS over other reflective LC devices is that the LCoS provides high performance, high-information-content microdisplays at significantly lower cost than competing technologies.
Currently, reflective TN LCDs have sufficient brightness and contrast for use in high definition projection applications. Normally black (NB) LC modes, since they offer higher contrast with low drive voltages as compared to normally white (NW) modes, are more readily adaptable for use in such applications. Projection systems utilizing transmissive LCDs have been able to obtain very high contrast ratios because the sheet polarizer and analyzer are separated (with inherently high contrast ratio greater than 1000:1) and not limited by a polarizing beamsplitter element ubiquitous in reflective projection optical systems. Contrast ratio is the ratio of the luminance of the bright state to the luminance of the dark state of the device. This polarizing beamsplitter element used in on-axis systems has limited acceptance angle. Consequently, system brightness and contrast are limited. In an off-axis reflective projection design, light input and output paths are spatially separated (like transmissive design) and a beam splitter cube is not required. One off-axis reflective projection design obtained total contrast ratio of greater than 400:1. (M. Bone et. al., SID 5th Annual Flat Panel Strategic Symposium, p81, 1998).
Furthermore, the contrast ratio decreases as the viewing angle increases due to the birefringent properties of the LC. Therefore, even an off-axis design has an inherent reduction in contrast as a function of the viewing angle. In order to increase brightness, the F# of the LCD system must be reduced (the aperture must be increased). The F# is defined as 1/(2 tan (xcex8)) where xcex8 is the half angle of the viewing cone. Unfortunately, a reduction in F# has a negative impact on the contrast of the system. Therefore, it is desirable to design an easily manufacturable TN LCD system that has improved contrast ratio performance without decreased optical performance and brightness in low F# or high brightness projection systems.
In projection systems utilizing reflective CMOS microdisplays or LCoS microdisplays where the polarizer and analyzer are separated (i.e. off-axis), retarder(s) or retardation film(s) or compensation film(s) are introduced in the output light path between the LCD and an analyzer, thereby yielding contrast ratios of greater than 500:1. The retarders function to alter the polarization of light reflected by the liquid crystal cell such that high contrast is obtained.
In accordance with one general aspect of the invention, there is provided a light valve for use in high contrast reflective microdisplays, comprising a twisted nematic mode reflective liquid crystal cell, a color filter positioned to accept non-polarized light incident to the light valve, a linear polarizer positioned between said color filter and said liquid crystal cell, an analyzer positioned in the path of the light reflected by said liquid crystal cell, and retarders positioned between said liquid crystal cell and said analyzer in the path of the light reflected by said liquid crystal cell. Light incident to the light valve is generally off-axis to said liquid crystal cell and said retarders function to decrease ellipticity and alter the polarization axis of light reflected by said liquid crystal cell.
In accordance with another general aspect of the invention, there is provided a light valve for use in high contrast reflective microdisplays, comprising a twisted nematic mode reflective liquid crystal cell, a color filter positioned to accept non-polarized light incident to the light valve, a linear polarizer positioned between said color filter and said liquid crystal cell, an analyzer positioned in the path of the light reflected by said liquid crystal cell; and a single retarder positioned between said liquid crystal cell and said analyzer in the path of the light reflected by said liquid crystal cell. Light incident to the light valve is generally off-axis to said liquid crystal cell and said retarder functions to decrease ellipticity and alter the polarization axis of light reflected by said liquid crystal cell.
In accordance with another general aspect of the invention, there is provided a method for improving the contrast of an off-axis light valve having a color filter, a linear polarizer, a twisted nematic mode reflective liquid crystal cell, and an analyzer. The polarization state of light after being reflected by said liquid crystal cell and before passing through said analyzer is determined. A first point representing a first polarization state of light reflected by said liquid crystal cell is plotted on a sphere (using the Poincare Sphere representation of the polarization state of light) and a retarder angle for a first retarder is chosen. A first retarder point is plotted on the sphere representing the chosen retarder angle. A first circle is then drawn on the surface of the sphere centered at the first retarder point and having along its radius said first point. A second point is determined as the intersection of said first circle with a plane passing through a line representing the linear polarization state of the analyzer. The retardation value of said first retarder is calculated as a function of the number of radians from said first point to said second point and the wavelength of light reflected by the liquid crystal cell. After passing through said first retarder, light will have a polarization state represented by said second point. A second retarder point representing a chosen retarder angle of a second retarder is plotted on said sphere. A second circle may then be drawn around said plane centered at said second retarder point. A third point is determined as the point along said second circle radius at the intersection of said second circle with said line representing the linear polarization state of the analyzer. The retarder value of the second retarder is calculated as a function of the number of radians from said second point to said third point and the wavelength of light reflected by the liquid crystal cell. The first and second retarders having chosen retarder angles and calculated retarder values are then placed between said analyzer and said liquid crystal cell in the path of light reflected by the liquid crystal cell.